In recent years, various types of polyimide resins have been used in many applications because they are superior in heat resistance, flame resistance, and chemical resistance, in addition to having superior electrical properties and superior mechanical strength. Specific applications include various components of electrical devices. For example, polyimide films have been used in flexible printed wiring circuit boards (“FPC” hereinafter), film carriers for tape automated bonding (“TAB” hereinafter), and chip-on-flex or chip-on-film (“COF” hereinafter). Other applications include a structural material for aircraft and the like.
One way of processing the polyimide film is etching. For example, in film carriers for FPC, TAB, and COF, a substrate is prepared from a film of polyimide. The polyimide film is partially etched to form through holes through the polyimide film of the substrate. In the application for a structural material for aircraft for example, the polyimide film is often chemically etched to provide a rough surface (surface roughing method), so as to improve surface properties such as surface wettability.
These and other etching methods and etchants have been proposed as etching techniques of the polyimide resin.
For example, Japanese Publication for Unexamined Patent Application No. 298974/1994 (Tokukaihei 6-298974; published on Oct. 25, 1994) discloses an etchant whose primary components are alkali hydroxide, hydrazine, and 1,3-dimethyl-2-imidazolidine. In another example, Japanese Publication for Unexamined Patent Application No. 195214/1998 (Tokukaihei 10-195214; published on Jul. 28, 1998) discloses an etchant that contains aliphatic alcohol, aliphatic amine, alkali metal compounds, and water. Further, Japanese Publication for Unexamined Patent Application No. 97081/1998 (Tokukaihei 10-97081; published on Apr. 14, 1998) discloses an etchant that contains oxyalkyl amine and alkali metal compounds.
All of these etchants disclosed in the foregoing publications allow the polyimide film to be etched quickly and desirably, regardless of the type of polyimide film. Note that, as the term is used herein, “desirable etching” means that the polyimide film is etched stably so that predetermined shapes and dimensions are obtained.
As the foregoing conventional examples indicate, the main focus of chemical etching of the polyimide film has been to improve the etchant.
It is recognized, however, that the foregoing conventional methods cause swelling on portions of the polyimide film in contact with the etchant. The swelling may cause working defects on the polyimide film.
Specifically, all of the foregoing conventional techniques etch the polyimide film using an alkali solution. Dipping the polyimide film in the alkali solution generates a gel-like substance on film surfaces. Throughout the specification of this invention, such a gel-like substance will be referred to as “swelling”.
The cause and composition of the swelling are assumed to be such that the swelling is:
(1) a substance that is generated by the reaction of the polyimide converting itself to polyamic acid in the presence of the alkali that opens the polyimide rings;
(2) a substance that is generated by the reaction of the polyimide converting itself to monomers by being decomposed by the alkali;
(3) a mixture of (1) and (2); and
(4) low-molecular-weight compounds that are generated when the main chains of the polyimide and polyamic acid are incised. However, no analysis has definitively proved these assumptions.
In etching the polyimide film to form through holes for example, the polyimide film is first masked with an alkali resistant resin or an alkali resistant metal. Etching is carried out after the mask has been removed only from portions of the polyimide film where the through holes are to be formed.
Such a method inevitably generates swelling on side faces of the through holes and end faces of the film because these portions of the polyimide film are not protected by the alkali resistant mask. Drying the film without completely dissolving and removing the swelling in the etching step and washing step causes the swelling to remain as a residue on the through holes and the end faces of the film. This causes working defects.
The working defects become more prominent as the diameter of the perforations of the polyimide film formed by etching become smaller. In other words, the working defects worsen when the process involves narrower polyimide films. Generally, the swelling (residue) that causes working defects generates in the form of a film or a thread at the perforated portions of the film where the upper and lower surfaces of the film meet. The presence of swelling therefore inhibits electrical conduction at the perforated portions and lowers reliability of the product.
One technique to avoid working defects that are caused by swelling is, for example, the etching method disclosed in Japanese Publication for Unexamined Patent Application No. 283486/1993 (Tokukaihei 5-283486; published on Oct. 29, 1993). This etching method removes polyamic acid (swelling) that results from etching. A problem of this method, however, is that it requires many steps and has poor productivity.
Beside the working defects at the perforations, etching causes another problem in the step of forming perforations, such as the through holes, in the manufacturing process of FPC for example. Namely, the polyimide film is curled (locally wrinkled) by the etchant, such as an alkali solution, which exposes the film. The curling of the film has detrimental effects on a yield in later processes.
Incidentally, an example of a substrate that uses the polyimide resin is a lead frame of an LOC (Lead on Chip) structure that enables high-density mounting. The lead frame of an LOC structure has been put to actual applications in the form of a prefabricated lead frame of a structure in which a film coated with an adhesive (such as thermoplastic adhesive or heat curable adhesive) is attached to the both sides of a heat-resistant polyimide film, so that semiconductor chips can be mounted thereon.
Conventionally, the polyimide film is attached to the lead frame by a stamping-and-attach method using a stamping mold. In this method, the polyimide film is stamped into strips and attached to the lead of the lead frame that is positioned below.
A drawback of this stamping-and-attach method is that flashes or chips of film are generated at the edges of the stamped film. This causes working defects.
For example, Japanese Patent No. 2923170 (Tokukaihei 6-334110; published on Dec. 2, 1994, registered on Apr. 30, 1999) discloses a technique to avoid working defects that are caused by stamping. Specifically, this publication discloses a polyimide film with a tear resistance of 50 kgf/20 mm to 70 kgf/20 mm, so as to provide a film having improved stamping workability.
However, the technique disclosed in this publication only improves workability of film stamping and the publication does not teach anything about improving etching workability using the alkali solution and at the same time improving stamping workability of the film.
To date, there has been no prior art technique that teaches improving etching workability using the alkali solution and at the same time improving stamping workability of the film.
The present invention was made in view of the foregoing problems and it is an object of the present invention to provide polyimide films that have superior workability, both in etching and stamping, and which effectively prevent working defects and improve productivity of each step.